Following the technology evolution, the target of the current electronic products is the low voltage for low power and the restricted error tolerance for greater component accuracy. For example, the circuits used to generate the reference quantity (a voltage or a current) must have a very low tolerance to ensure the reference quantity to have a precise and stable value.
The problem of obtaining a very low tolerance is generally solved by providing trimming elements in the integrated device to connect to the circuit in order to correct an inaccuracy thereof. During the trimming process, the characteristic parameter of the circuit element is measured and compared with a nominal value thereof. The trimming elements are activated or deactivated according to the result of this comparison so as to obtain the required tolerance.
Moreover the market constraints imply to supply these components at lower price and smaller package but with a constant pin out.
So there is a need to improve the circuit architecture and the trimming techniques to increase the accuracy and the stability of these components over time and temperature. Today, conventional trimming is performed before packaging, on the wafer (EWS: Electrical Wafer Sort). In fact, the EWS test is performed on the wafer prior to severing the individual integrated circuit die destined to final packaging operations. This needs an additional step in the production cycle together with internal extra pads associated to the trimming elements.
Trimming may even need to be carried out at the packaged circuit device final testing stage. Indeed, the isolation resin injected into the mold exerts a pressure on the semiconductor chip that may cause mechanical stress/strain on the crystalline semiconductor capable of altering electrical parameters. For this reason, it may be necessary to carry out a trimming operation when the packaged circuit device is tested.
An example of a device requiring a trimming at the very late stage of the manufacturing process, that is after completion of the packaging of the integrated circuit device incorporating the device itself, is a band-gap regulator for producing a constant voltage reference insensitive to the working temperature.
The trimming elements may consist of the use of metal fuses. The circuit element is formed by several elemental components connected to each another, for example, in series. A trimming element in series with a metal fuse is connected in parallel with each elemental component. The terminals of each metal fuse are accessible from outside the integrated circuit by means of a couple of firing pads. If a current of high value is supplied to the firing pads, the metal fuse is blown; the corresponding trimming element is thus deactivated (since it is disconnected from the elemental component).
The trimming element may consist in the use of Zener diodes in the place of the metal fuses. If a current of high value is supplied to the firing pads, the Zener diode is blown and it is brought to a breakdown condition wherein it behaves as a short-circuit; the corresponding trimming element is thus activated (since it is connected to the elemental component).
In these solutions the firing pads take up a considerable amount of space on a semiconductor chip wherein the integrated circuit is formed. This leads to the need to limit the number of firing pads and hence trimming elements, correspondingly limiting the degree the accuracy which can be obtained.
The use of these solutions in an integrated circuit encapsulated in a package requires too great a number of external pins (connected to the firing pads) and metal connection tracks which are very wide in order to be able to withstand the current necessary to blow the metal fuses and the Zener diodes. The trimming process may not correct variations in the characteristic parameter of the circuit element caused by mechanical and thermal stresses undergone by the integrated circuit during encapsulation operation.
In view of the foregoing, there is a need to provide a trimming circuit for an integrated device having a reduced number of components and requiring no dedicated pin or only a dedicated pin.